It's easy to point at any one thing and make that the cause. More likely though it's a combination of things all working together.
Yes, the labor market is changing. As the economy changes from industrial to services, so demands for some jobs go up, others go down. The emphasis on automation (AI being just another card in that deck) means that more is produced, while employing ever fewer people.
100 years ago agriculture consumed 27% of the work force. Today its 1%, while at the same time production is much higher.
The explosion of student debt, the access to knowledge online (outside of a college course), the declining birth rate, the current hostility to foreign students, the poor image of the US abroad, all contribute though. Each factor is small in isolation, but together they're moving the needle.
The saturation of the professional class also plays a role. Do we need to churn out several thousand new architects every year? Especially into a society that sees the concept of architecture as irrelevant?
Yes, colleges will close. The demand caused by the boomers will go away. But that's part of a much bigger shift in US society.